I had a 4010 and although I did not run it a lot other than a short test run, I'm not surprised that it generally seems slow as it is a very heavy loco. I christened it "the electric brick"!
As regards stuttering, there are a number of possible reasons.
1. Batteries getting low on charge, although this usually shows itself when trying to run the loco in reverse.
2. Running two RC locos together, even on different frequencies. The 25-27 kwz frequency Playmobil used for its RC trains is not the best range and so as a train on one frequency gets near a controller using another frequency this can affect the signal received by the train and cause it to judder and maybe change speed too. I notice that you're running another train at the same time, but can't tell whether it is another RC one or one running off a mains transformer.
3. Other electrical equipment can interfere with the loco's receiver, e.g. someone using a mobile 'phone near the loco. This, again, gvies juddering, for example. If the other train you're running is running on a mains transformer, then the transformer may be causing the problems. Does the train always speed up or slow down at the same point on the track? If so, is that near to the transformer?
As for the humming, this is quite usual. I've just switched on the controller of one of my steam locos and, with the control set in the middle position the motor in the loco hums away as it is receiving a signal from the transmitter.
